Geographic Range

Shadow chipmunks, Tamias senex, occur in California, Oregon, and Nevada. The species ranges northward from the central Sierra Nevadas in California, skirting Nevada near Lake Tahoe, and into the eastern Cascade Range of Oregon. In northern California, this species is found from the Pacific Coast to the Warner mountains between the Eel and Klamath rivers.

In the Sierras, shadow chipmunks are limited to elevations above 1,500 m. They do not move to lower elevations on the eastern or western slopes of the range, where other species of chipmunk occur. (Gannon and Forbes, 1995; Verts and Carraway, 1998)

Physical Description

Tamias senex is a relatively large chipmunk. The total length of T. senex is between 229 and 261 mm, and the hind foot measures 34 to 38 mm. As is the case for most members of the genus Tamias, females are slightly larger than males, with an average dimorphism ratio of 1.033. Mass of females ranges between 73.0 g and 108.5 g, compared to males, which measure 66.8 g to 99.3 g. Mean head and body lengths also differ between sexes, with females measuring 144.12 mm, compared to 139.54 mm for males.

The pelage of inland popluations of T. senex varies seasonally in color, length, and quality. In both the winter and summer, pelage is white ventrally, gray on the top of the head and the rump, and has a marked black dorsal stripe. Two molts occur annually. The winter pelage is replaced by a summer molt just after breeding activity in spring. The summer molt begins at the head and continues toward the rump, resulting in short, coarse, brightly-colored pelage. The winter molt occurs in September, October, and November (just before hibernation) beginning at the rump and continuing toward the head.
The winter molt results in long, dense, silky, dull-colored pelage with gray upper parts. (Gannon and Forbes, 1995; Sutton and Patterson, 2000)

Tamias senex is polymorphic. Coastal populations are larger and darker in color, whereas inland populations are smaller and paler. The transition from the dark pelage of coastal specimens to the paler pelage of inland specimens is quite abrupt about 32 km inland from the Pacific coast, near the eastern margin of the coastal forest. This variation in pelage may be an adaptive response which makes these chipmunks more cryptically colored in different habitats. (Sutton and Patterson, 2000)

Genital bones of each sex are diagnostic of T. senex, and allow these animals to be distinguished from Tamias orchrogenys, a species with which it is often confused. Compared to T. orchrogenys, the baculum is longer (3.09 mm versus 2.47 mm) and more slender. (Gannon and Forbes, 1995)

Chipmunks are endothermic. Because they enter hibernation, their body temperature varies, making them heterothermic. However, they may also be considered homoiothermic. Individuals maintain their body temperature around set points; higher when they are active, and lower when they are torpid. (Nowak, 1999; Wilson and Ruff, 1999)

Reproduction

The mating system of this species has not been described. However, many other species in the genus Tamias are polygynous. Males may travel extensively during the breeding season, seeking females with whom to mate. It is likely that T. senex is similar. (Wilson and Ruff, 1999)

Young animals start to reproduce in the breeding season following their birth. Mating season begins in mid-March, about one month after these chipmunks emerge from hibernation, and continues for about four weeks. Populations found in the Great Basin begin their mating season earlier than populations in the Sierra Nevadas and the Volcanic Plateau.

As the breeding season approaches, male reproductive organs increase dramatically in size. Testis length doubles, and the prostate and cauda epididymis triple in width. In addition, both the diameter of the bulbo-urethral glands and the length of the seminal vesicles increase five times. Although the swelling and post-breeding reduction of the testes are gradual, changes in the bulbo-urethral glands and semial vesicles occur abruptly. (Gannon and Forbes, 1995)

In females, the vulva perforate at the onset of estrus. The genital region becomes swollen, and the uterine horns are inflated and twisted. Nipples become enlarged during estrus and pregnancy, and reach maximum size during lactation. (Gannon and Forbes, 1995)

Breeding occurs in March and April in most locations, although some geographic variation in timing of breeding exists.

Range number of offspring

3 to 5

Average number of offspring

4.5

Average gestation period

28 days

Average weaning age

2 months

Average age at sexual or reproductive maturity (female)

10 months

Average age at sexual or reproductive maturity (male)

10 months

Details on the parental investment of these animals are not available. However, in other species of Tamias, males provide no parental care. Tamias senex is altricial at birth. Females provide all protection and care to the young. Young usually disperse in the late summer or autumn of their birth year. (Nowak, 1999; Wilson and Ruff, 1999)

Lifespan/Longevity

Members of the subgenus Neotamias are reported to live as long as 8 years in the wild, and may reach almost 11 years of age in captivity. Although there are no firm data for T. senex, we may assume that they are similar to other members of their subgenus. (Nowak, 1999)

Behavior

Tamias senex is generally social. These chipmunks are diurnal, and hibernate from about November through March. Tamias senex is one of the few species in North America which puts on a layer of fat to survive the winter months. Seasonal fat accumulation increases the weight of individuals as much as 20% before November. They experience fewer bouts of wakefulness during hibernation than do other members of the genus. (Gannon and Forbes, 1995; Hartson, 1999; Harvey and Polite, 2005)

These chipmunks are largely arboreal, and like other members of their subgenus, build nests in trees. Such nests may be located as high as 23 m in the trees. Other western chipmunks may also occupy night or winter nests in rocky ground locations. Although data for this species are not available, they are likely to resemble other arboreal chipmunks of western North America in this regard. (Nowak, 1999)

Home Range

Home ranges for Tamias amoenus, a related species, are recorded at 1.6 hectares for males and 0.93 hectares for females. It is likely that T. senex occupies similar home ranges.

Communication and Perception

Tamias senex show differences in call notes compared to other chipmunks, like T. sonomae. Unlike other species, T. senex does not produce a whistle vocalization. Tamias senex produces an alarm call in 3 to 5 metallic barks with maximum frequency 12.98 kHz and minimum frequency 4.56 kHz. The average call duration is 0.81 seconds, followed by a single chirped note. (Gannon and Forbes, 1995; Hartson, 1999)

In addition to vocal communication, these diurnal mammals use visual cues, such as body posture, in their interactions. During mating season, there are visual changes in female genitalia, indicating that males may use such cues in finding mates. Tactile communication between a mother and her young undoubtedly occurs in the nest, and may play some role in mating behaviors. The role of chemical communication has not been studied in these animals. (Nowak, 1999)

Food Habits

Tamias senex is primarily herbivorous, spending the majority of its time collecting and storing food. Diet consists mainly of the seeds of various herbs and grasses, but also contains fungi, fruits of shrubs and forest trees, and occasional insects. Some examples of food choices are acorns, chinquapins, spruces, hazelnuts, cherries, hemlocks, and berries. (Sutton and Patterson, 2000; Verts and Carraway, 1998)

Predation

Natural predators including coyotes, badgers, weasels, snakes, foxes, and large birds of prey, have the most influence on the population of T. senex. Owl attacks are also a threat, although not very common because these chipmunks are usually diurnal, whereas owls are usually nocturnal. (Owca, 2005)

Ecosystem Roles

Most small rodents, including the T. senex, play important roles in their ecosystem. Besides being an abundant food source for their predators, they are also an important biotic vector in seed dispersal. They may provide habitat for certain parisites. (Owca, 2005)

Ecosystem Impact

disperses seeds

Economic Importance for Humans: Positive

Direct impact of this species on humans is limited. Tamias senex may have an indirect economic importance to humans as a result of their seed dispersal. (Owca, 2005)

Economic Importance for Humans: Negative

When T. senex populations become too dense, this species may become a pest. Chipmunks can damage pine seeds, preventing reforestation. This may cause negative economic consequences to humans. (Owca, 2005)

Conservation Status

Although the range of this species is not large, these chimpmunks are not thought to be at great risk.

Other Comments

The genus name Tamias is derived from the Greek word tamias, meaning steward. The species name senex means dim or shadowy, in Latin. Common names for T. senex include Allen's chipmunk, gray chipmunk, California chipmunk, Was-La of the Klamath, large mountain chipmunk, and shadow chipmunk. Shadow chipmunk was most recently used by Jameson and Peeters (1988), referring to the dark coloration of the pelage and the preference of this species for densely shaded areas of the forest. (Gannon and Forbes, 1995)

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal

active during the day, 2. lasting for one day.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

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The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.